Tribal,
What is a typical location of your cg when you do this? The helicopter attitude may become more level with the aft right wind, which may show a slight decrease in required mast torque. However, I doubt the magnitude of this effect will be as large as what you're finding.
I think the most likely area for the savings is the power requirement of the tail rotor. The aft right wind may direct the tail rotor wash away from the vertical stab, or even around it in such a way that favourable yaw forces are generated. Again 5-10% seems high, since I would have thought that would represent the total power requirement of the tail rotor.
One caution, everything you're saying suggests you only get this excess power with a tail wind. As you translate forward, you're necessarily eliminating that excess, and arriving at a minimum wind condition (also max power required) with a rate of descent established. You'd be better off starting at that max power condition being level or with a rate of climb.
Just my thoughts,
Matthew.